Plex NAS for streaming 1080p and 4K vids

New to NAS’s, will be my first one. I’m mainly interested in the ability to play 1080 vids through plex, perhaps 4K one day. Would not streaming more than 3 files at a time, most likely.
Recommendations? I feel like some of the Plex recommended NAS may be more than I need, as far as providing other services I would not use (video surveillance). Around $500 diskless, at least 8 TB, but expandable perhaps.

Simple video playback is not very resource demanding task. The DS718+ or DS918+ will not only stream 1080p and 4K but also transcode it in local DS Video app. Plex trancoding will be limited to 1080p (5 and more streams) and will struggle with transcoding a 4K video. But simple streaming will work on Plex with 1080p and 4K files. DS718+ can be expanded to 7 drives and DS918+ to 9 drives if you ever run out of the space.

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